1. Field of the Invention
The invention pertains to storage-stable foamable compositions based on epoxy resins. More specifically, specific combinations of blowing agents and curing accelerators are utilized to obtain resins which exhibit improved stability at room temperature over prolonged periods of time, yet can be heated and cured to provide foams capable of strengthening and stiffening hollow metal parts and the like.
2. Discussion of the Related Art
It is known that a number of industries, e.g., the automobile industry, require parts that are both strong and light-weight. One attempt to achieve this balance between strength and minimal weight provides for hollow metal parts. However, hollow metal parts are easily distorted. Accordingly, it is also known that the presence of structural foam in the cavities of the hollow parts can improve the strength and stiffness of such parts.
Generally, such foams comprise a thermosettable resin such as an epoxy resin, a blowing agent and a filler. Preferably, these foams have a density of about 20-40 lb/ft3 (about 0.30-0.65 g/cc) and are able to withstand heat in excess of 175xc2x0 C., more preferably in excess of 200xc2x0 C. Optional ingredients include accelerators, curing, agents, processing aids, stabilizers, colorants, and UV absorbers.
Specific formulas for structural foam can vary widely. For example, U.S. Pat. No. 5,575,526 teaches several resin-based structural foams including Formula 2, which contains 54.5% EPON 828 epoxy resin, 7.5% HALOXY 62 epoxy diluent, 6.1% DER 732 flexible epoxy, 2.0% EXPANCEL 551DU blowing agent, 8.8% MICROS microspheres, 17.7% 3M K20 microspheres and 3.4% DI-CY dicyandiamide curing agent. U.S. Pat. No. 5,755,486 discloses thermally expandable resin-based materials containing, for example, epoxy resin, acrylonitrile-butadiene rubber, calcium carbonate, carbon black, fumed silica, glass spheres, curing agent, accelerator, and blowing agent. Modified ureas and blocked boron trichloride complexes are taught to be suitable for use as accelerators. Structural foams such as, e.g., TEROCORE(copyright) (a product of Henkel Surface Technologies), are now used in a variety of industries.
One recurrent problem with many of the structural foam formulations of this type that have been developed to date, particularly the one part formulations, has been their relatively short shelf-life.
Ideally, the components of such formulations once combined together should remain stable and unreacted under the conditions normally encountered during storage and shipment. It would be highly desirable for the properties of the formulations to be unchanged when stored at temperatures at or somewhat above room temperature over a several week or several month period. Unfortunately, certain of the presently available structural foam products tend to partially expand and cure at a temperature of 130xc2x0 F. (a temperature which storage facilities and transportation means frequently exceed during the summer months or in warm climates) within 24 hours. Merely lowering the amount of curative is not the solution to the problem, since this will typically adversely affect the ability to cure and foam the resin when desired within commercially acceptable temperature and time limits.
It has now been surprisingly discovered that the shelf-life problems inherent in previously known structural foam formulations based on epoxy resins may be effectively alleviated through the use of amine-epoxy adduct curatives and an azodicarbonamide blowing agent. Storage-stable foamable compositions thus are provided which are comprised of one or more epoxy resins, one or more blowing agents (at least one of which must be an azodicarbonamide), and one or more curatives (at least one of which must be an amine-epoxy adduct). In one especially advantageous aspect of the invention, the foamable composition is in the form of a pliable dough which additionally contains one or more fillers, particularly hollow glass microspheres. Such compositions when foamed and cured furnish reduced density foams having excellent physical properties, including high strength and stiffness.
Any of the thermosettable resins having an average of more than one (preferably, two or more) epoxy groups per molecule known or referred to in the art may be utilized as the epoxy resin component of the present invention.
Epoxy resins are described, for example, in the chapter entitled xe2x80x9cEpoxy Resinsxe2x80x9d in the Second Edition of the Encyclopedia of Polymer Science and Engineering, Volume 6, pp. 322-382 (1986). Exemplary epoxy resins include polyglycidyl ethers obtained by reacting polyhydric phenols such as bisphenol A, bisphenol F, bisphenol AD, catechol, resorcinol, or polyhydric alcohols such as glycerin and polyethylene glycol with haloepoxides such as epichlorohydrin; glycidylether esters obtained by reacting hydroxycarboxylic acids such as p-hydroxybenzoic acid or beta-hydroxy naphthoic acid with epichlorohydrin or the like; polyglycidyl esters obtained by reacting polycarboxylic acids such as phthalic acid, tetrahydrophthalic acid or terephthalic acid with epichlorohydrin or the like; epoxidated phenolic-novolac resins (sometimes also referred to as polyglycidyl ethers of phenolic novolac compounds); epoxidated polyolefins; glycidylated aminoalcohol compounds and aminophenol compounds, hydantoin diepoxides and urethane-modified epoxy resins. Mixtures of epoxy resins may be used if so desired; for example, mixtures of liquid (at room temperature), semi-solid, and/or solid epoxy resins can be employed. Any of the epoxy resins available from commercial sources are suitable for use in the present invention. Preferably, the epoxy resin has an epoxide equivalent molecular weight of from about 150 to 1000. The use of epoxy resins based on glycidyl ethers of bisphenol A is especially advantageous. The epoxy resin(s) preferably have an average of about 2 epoxy groups per molecule. The epoxy resin or resins should be selected so as to provide the desired combination of properties in both the foamable dough and the final cured foam.
Selection of the blowing agent system to be employed is critical to the attainment of a storage-stable foamable composition. Azodicarbonamide (also sometimes referred to as 1,1xe2x80x2-azobisformamide, AZDC, or ADC) must comprise at least part of the blowing agent system. Preferably, the blowing agent system is predominantly (i.e.  greater than 50% by weight) azodicarbonamide. Even more preferably, at least 80% by weight of the total amount of the blowing agent system is azodicarbonamide. Blowing agents such as sulfonyl hydrazides may be used as minor components of the blowing agent system. In one embodiment, however, azodicarbonamide is essentially the only blowing agent present in the foamable composition. Azodicarbonamide is available from a number of commercial sources; for example, it is sold under the trademark UNICELL by Dong Jin Chemical of South Korea and under the CELOGEN trademark by Uniroyal Chemical. The particle size of the azodicarbonamide may be adjusted so as to provide the desired foaming characteristics in the foamable composition. For example, it has been found that relatively small particle size azodicarbonamides tend to produce foams having more uniform cell structure than coarser grade azodicarbonamides. xe2x80x9cActivatedxe2x80x9d or xe2x80x9cmodifiedxe2x80x9d forms of azodicarbonamide may be used to advantage in the present invention.
In some formulations, it may be desirable to also utilize a blowing agent activator or accelerator so as to lower the temperature at which release of the gas from the blowing agent takes or to increase the rate at which gas is released at a given cure temperature. Typically, from about 0.1% to about 2% blowing agent accelerator/activator based on the weight of the foamable composition is employed, although the optimum amount will of course vary depending upon the activator/accelerator selected, the amount of blowing agent, cure temperature and other variables. Excess activator/accelerator should not be present in the foamable composition, however, since the storage stability may be undesirably compromised. Suitable blowing agent activators/accelerators include, but are not limited to, ureas (such as the surface-coated oil-treated urea sold by Uniroyal Chemical under the trademark BIK OT), polyols, organic acids, amines, and lead, zinc, tin, calcium and cadmium oxides and salts (including carboxylic acid salts).
Also critical in the operation of the present invention is the type of curative system used. Although more than one type of curative may be present, at least one curative must be an amine-epoxy adduct. Amine-epoxy adducts are well-known in the art and are described, for example, in U.S. Pat. Nos. 3,756,984, 4,066,625, 4,268,656, 4,360,649, 4,542,202, 4,546,155, 5,134,239, 5,407,978, 5,543,486, 5,548,058, 5,430,112, 5,464,910, 5,439,977, 5,717,011, 5,733,954, 5,789,498, 5,798,399 and 5,801,218, each of which is incorporated herein by reference in its entirety. Such amine-epoxy adducts are the products of the reaction between one or more amine compounds and one or more epoxy compounds. Carboxylic acid anhydrides, carboxylic acids, phenolic novolac resins, water, metal salts and the like may also be utilized as additional reactants in the preparation of the amine-epoxy adduct or to further modify the adduct once the amine and epoxy have been reacted. Preferably, the adduct is a solid which is insoluble in the epoxy resin component of the present invention at room temperature, but which becomes soluble and functions as an accelerator to increase the cure rate upon heating. While any type of amine could be used (with heterocyclic amines and/or amines containing at least one secondary nitrogen atom being preferred), imidazole compounds are particularly preferred. Illustrative imidazoles include 2-methyl imidazole, 2,4-dimethyl imidazole, 2-ethyl-4-methyl imidazole, 2-phenyl imidazole and the like. Other suitable amines include, but are not limited to, piperazines, piperidines, pyrazoles, purines, and triazoles. Any kind of epoxy compound can be employed as the other starting material for the adduct, including monofunctional, bifunctional, and polyfunctional epoxy compounds such as those described previously with regard to the epoxy resin component. Suitable amine-epoxy adducts are available from commercial sources such as Ajinomoto, Inc., Shell, Pacific Anchor Chemical Company, and the Asahi Chemical Industry Company Limited. The products sold by Ajinomoto under the trademarks AJICURE PN-23 and AJICURE PN-40 and by Air Products under the trademark ANCAMINE 2441 are especially preferred for use in the present invention.
The curing of the epoxy resin(s) utilized in the present invention may be additionally assisted by the incorporation of other substances capable of promoting the desired hardening upon heating. Such materials are referred to herein as xe2x80x9ccurativesxe2x80x9d, but also include the materials referred to by workers in the field as curing agents, hardeners, accelerators, activators, and catalysts. While certain curatives promote epoxy resin curing by catalytic action, others participate directly in the reaction of the resin and become incorporated into the thermoset polymeric network which is formed. Although any of the curatives (curing agents) known in the epoxy resin field (see the chapter in the Encyclopedia of Polymer Science and Engineering referenced hereinabove) may be used in the present invention in addition to the amine-epoxy adduct, the use of one or more nitrogen-containing compounds such as imidazoles, substituted ureas, amino compounds, amine salts, and quaternary ammonium compounds as the auxiliary curative(s) is desirable (provided that such compounds which cause an unacceptably high degree of epoxy resin reaction under normal storage conditions are avoided). Dicyandiamide (sold commercially by Air Products under the trademark xe2x80x9cDICYxe2x80x9d) is a particularly preferred auxiliary curative, although other guanidine compounds may also be utilized. In one desirable embodiment of the invention, dicyandiamide (preferably, about 0.5-8 wt. % based on the total weight of the foamable composition) is used in combination with an amine-epoxy adduct (preferably, about 0.1-5 wt. %) in the curative system. The curative system must, however, be selected such that it does not catalyze curing of the foamable composition to any significant extent under typical storage conditions over an extended period. Preferably, the amounts and identities of the components of the curative system are selected such that the foamable composition retains a workable consistency (in one embodiment of the invention, a consistency resembling that of a pliable dough or puffy) for at least two weeks in storage at 130xc2x0 F. (54xc2x0 C.) without a significant loss in cured foam properties and does not expand in volume or decrease in specific gravity to an unacceptable extent under such conditions, but foams and cures within about 10 minutes upon being heated at about 150xc2x0 C.
It will be especially desirable to include one or more glass fillers in the foamable composition, as such fillers have been found to impart useful characteristics to the resulting foam (especially where it is to be used to reinforce a structural member). For example, hollow glass microspheres may be added to reduce the density of the foam, and thus the overall weight of the reinforced structural member, while maintaining good stiffness and strength. Commercially available hollow glass microspheres (sometimes also referred to as glass microballoons or microbubbles) include the materials sold by Minnesota Mining and Manufacturing under the trademark SCOTCHLITE, with suitable grades including those sold under the designations B38, C15, K20 and VS 5500. The glass microspheres preferably have diameters in the range of from about 5 to about 200 micrometers (more preferably, less than about 70 micrometers). The crush strength of the hollow glass microspheres may be selected in accordance with the desired characteristics of the cured thermoset foam or reinforced structural member containing said foam. Microsperes having a crush strength in excess of 500 psi are typically employed, however. In a particularly desirable embodiment, hollow glass microspheres comprise from about 5 to about 50 percent by weight of the thermosettable composition. Glass fiber is another preferred type of glass filler, since it helps increase the strength and stiffness of the resulting foam. The glass fiber may be chopped, milled or in other physical forms.
Other types of fillers may also be optionally present in the foamable composition. Any of the conventional inorganic or organic fillers known in the thermosettable resin art may be used including, for example, silica (including fumed or pyrogenic silica, which may also function as a thixotropic or rheological control agent), calcium carbonate (including coated and/or precipitated calcium carbonate, which may also act as as thixotropic or rheological control agent, especially when it is in the form of fine particles), fibers other than glass fibers (e.g., wollastonite fibers, carbon fibers, ceramic fibers, aramid fibers), alumina, clays, sand, metals (e.g., aluminum powder), microspheres other than glass microspheres (including thermoplastic resin, ceramic and carbon microspheres, which may be solid or hollow, expanded or expandable), and any of the other organic or inorganic fillers known in the epoxy resin field. Other optional components include diluents (reactive or non-reactive) such as glycidyl ethers, glycidyl esters, acrylics, solvents, and plasticizers, toughening agents and flexibilizers (e.g., aliphatic diepoxides, polyaminoamides, liquid polysulfide polymers, rubbers including liquid nitrile rubbers such as butadiene-acrylonitrile copolymers, which may be functionalized with carboxy groups, amine groups or the like), adhesion promoters (also known as wetting or coupling agents; e.g., silanes, titanates, zirconates), colorants (e.g., dyes and pigments such as carbon black), stabilizers (e.g., antioxidants, UV stabilizers), and the like.
The formulations of the present invention preferably contain the above-discussed components in the following amounts (expressed as a percentage of the total weight of the foamable composition).
Preferably, epoxy resin is present in an amount which is greater than the amount of any of the other components in the foamable composition.
The above-described ingredients may simply be combined and mixed to form the foamable composition using any of the conventional processing methods known in the epoxy resin art.
The preferred finished product preferably has the consistency of dough for easier handling. The dough may be shaped by extrusion or by hand or other means into any desired configuration. For example, the dough may be conformed to follow the contours of an irregular surface. The dough can then be cured and foamed by heating, preferably at a temperature of at least about 250xc2x0 F. (about 120xc2x0 C.), more preferably, at least about 300xc2x0 F. (about 150xc2x0 C.). Preferably, it is placed on or near the surface of a solid article comprised of, for example, metal or a thermoset or thermoplastic polymer such that a composite of the structural foam adhered to said surface is created upon foaming and curing. Alternatively, of course, the dough can be placed in a suitable container or vessel for storage until the use of the foamable composition is desired.
The foamable compositions of the present invention may be utilized in any end-use application where a relatively light-weight, yet strong, thermoset foam is needed. However, the foamable compositions are especially useful in the production of automobiles and other vehicles to maintain or increase the strength of structural members such as rockers, pillars, radiator support beams, doors, reinforcing beams, and the like. The use of structural foams in such applications is described, for example, in U.S. Pat. Nos. 4,901,500; 4,908,930; 4,751,249; 4,978,562; 4,995,545; 5,124,186; 5,575,526; 5,755,486; 4,923,902; 4,922,596; 4,861,097; 4,732,806; 4,695,343; and 4,610,836 (each of which is incorporated herein by reference in its entirety).